Reuse PanelToplevel window as actual panel class.

Reuse PanelToplevel window as actual panel class.

This way we can:
1) spare destroying function, just use gtk_widget_destroy();
2) simplify access Plugin->Panel to gtk_widget_get_toplevel();
3) simplify access toplevel window of Panel to GTK_WINDOW(p).
Old Panel is priv mamber of new LXPanel class. No extra memory consumed
because we can remove topgwin member from Panel structure later.